Magnetoresistive sensor employing an exchange-bias enhancing layer with a variable-composition transition region

ABSTRACT

An exchange-biased magnetoresistive (MR) read transducer in which the MR layer composition is changed at the interface with an antiferromagnetic layer, which is in direct contact with the ferromagnetic MR layer. The exchange-bias field strength H UA  in the MR layer is increased at room temperature by adding a specially-optimized transition region in the ferromagnetic MR layer at the interface. The percentage of iron in the ferromagnetic alloy varies from a higher value at the interface to a lower value at the opposite end of the transition region. The higher iron ratio at the antiferromagnetic interface enhances the exchange-bias field H UA  and the lower iron ratio throughout the bulk of the ferromagnetic MR layer maintains the lower coercivity preferred in the layer, thereby enhancing the longitudinal bias field with respect to the MR coercivity. Advantageously, the enhanced longitudinal bias effect of the special ferromagnetic transition region does not reduce the critical temperature T cr  at which the temperature-dependent exchange-bias field H UA  (T) approaches zero.

This application is a continuation of application Ser. No. 08/174,748,filed Dec. 29, 1993, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to magnetoresistive (MR) sensors forreading data signals stored in magnetic media and specifically to a MRsensor with a special ferromagnetic transition layer for improvedexchange-biasing of the ferromagnetic sensing layer.

2. Description of the Related Art

In magnetoresistive (MR) sensors, it is important to provide magneticbias to suppress domain formation in the MR film element. A MR sensordetects magnetic field signals through the resistance changes in a thinfilm ferromagnetic MR strip arising from changes in external magneticflux. Two magnetic bias fields are usually preferred for optimal MRelement operation. A transverse bias field is usually provided to biasthe MR strip so that it exhibits a linear response to external magneticflux. This transverse bias field is normal to the plane of the magneticrecording medium and parallel to the surface of the MR strip. The secondbias field is a longitudinal bias field that extends parallel to thesurface of the magnetic medium and parallel to the lengthwise dimensionof the MR strip. This longitudinal bias operates to suppress magneticdomain formation. Troublesome discontinuous changes in sensitivity andlinearity occur in the outputs of thin-film MR sensors because thesensor dimensions are of the same order as domain dimensions. Thesediscontinuities are known in the art as "Barkhausen noise" and are theresult of sudden chaotic changes in domain wall positions with changesin applied magnetic field. The two simplest methods for avoidingBarkhausen noise in MR sensors are (a) to eliminate all domain walls or(b) to force such domain walls to be immobile.

A useful approach to eliminating domain walls in thin-film MR strips isto force the strip into a single domain by applying a permanent externalmagnetic bias. The related art is replete with useful methods forproviding such external magnetic bias to thin-film MR strips. Oneapproach to obtaining unidirectional anisotropy in the MR strip isthrough exchange interaction at the atomic boundary between anantiferromagnetic material and the ferromagnetic material making up theMR strip. Exchange anisotropy is well-known in the art as a form ofsurface anisotropy located at the phase boundary between a ferro- orferrimagnet and an antiferromagnet. For instance, exchange anisotropy isknown to occur on cobalt particles with an antiferromagnetic cobaltmonoxide surface layer. The exchange coupling of the last plane of themagnetically-fixed antiferromagnetic lattice to the first ferro- orferrimagnetic lattice plane leads to unidirectional vector anisotropy.This vector anisotropy behaves like a dc-bias field that displaces thehysteresis loop along the H axis and causes a finite anhystereticmagnetization in zero external field.

In U.S. Pat. No. 4,103,315, Robert D. Hempstead, et al. disclose atechnique for minimizing domain walls in thin-film magnetic transducersthat relies on the magnetic biasing effect of exchange anisotropy.Hempstead, et al. provide extensive detailed discussion of thin-filmmagnetic materials and fabrication methodology related to exchange biasapplications and their patent is entirely incorporated herein by thisreference.

Subsequent to the work by Hempstead, et al., many practitioners haveproposed refinements to the exchange-biasing technique to incrementallyimprove MR sensor performance. In U.S. Pat. No. 4,663,685, Ching H.Tsang discloses a MR read transducer assembly in which the thin-film MRlayer is longitudinally biased by exchange anisotropy only in the endregions. The bias field is developed by a thin film of antiferromagneticmaterial deposited in direct contact with the MR layer over the endregions. Limiting the longitudinal bias field to the end regions permitsa central transverse bias field to maintain the central region of the MRlayer in a linear response mode.

In U.S. Pat. No. 4,639,806, Toru Kira et al. disclose a thin-filmmagnetic sensor strip that is exchange-coupled to an adjacentpermanently-magnetized ferromagnetic layer of higher coercivity toprovide a longitudinal bias consisting of a combination of permanentmagnetic field and exchange-bias field.

U.S. Pat. No. 4,713,708, issued to Mohamad T. Krounbi et al., disclosesan exchange-biased MR sensor assembly that includes a third thin layerof soft magnetic material, where the antiferromagnetic exchange-biasinglayer is removed in the middle region of the MR strip leaving only thethin film of soft magnetic material separated from the MR layer in thecentral region only by a decoupling layer that interrupts the exchangecoupling so that traverse bias is produced only in the central regionupon connection of a bias source to conductor leads, which are connectedto the MR strip within the end region.

In U.S. Pat. No. 4,782,413, James K. Howard et al. disclose a MR sensorthat uses an iron-manganese (FeMn) alloy as the antiferromagneticexchange-biasing layer. The presence of the body-centered-cubic alphairon-manganese alloy improves the longitudinal exchange bias in theferromagnetic MR layer.

In U.S. Pat. No. 4,785,366, Mohamad T. Krounbi et al. disclose a MR readtransducer that is exchange-biased over its entire length by acontinuous thin film of antiferromagnetic material with a thin film ofsoft magnetic material disposed in the passive end regions such that thebias directions in different regions of the bias film are defined toproduce optimum device performance. Krounbi et al. initialize theexchange-biasing layer to produce an effective bias field that isdirected substantially longitudinally within the passive end regions andat some selected angle within the active central region of the MR sensorlayer. Thus, the exchange-biasing layer is used to produce both thelongitudinal and transverse bias fields.

In U.S. Pat. No. 4,809,109, James K. Howard et al. disclose an improvedMR read transducer having an exchange-biased MR layer that is subjectedto a thermal annealing process to create a ternary antiferromagneticalloy at the junction between the ferromagnetic and antiferromagneticlayers. The ternary alloy provides the desired exchange-bias field atroom temperature and exhibits an unusually high ordering temperature.Howard et al. neither consider nor suggest forming avariable-composition alloy layer in the ferromagnetic element to improveexchange-bias field levels, restricting their discussion to forming anew antiferromagnetic alloy at the interface between the two originalfilms.

In U.S. Pat. No. 4,825,325, James K. Howard discloses a MR sensor thatis longitudinally biased by the exchange anisotropy formed between theMR layer and a very thin layer of antiferromagnetic material, where theentire structure is covered with a protective film to prevent oxidationdamage to the materials during subsequent thermal cycling.

In U.S. Pat. No. 4,967,298, Greg S. Mowry discloses an elongated MRsensor strip that is longitudinally biased to maintain a single domainsense region using exchange-biasing material atomically coupled to thestrip at the ends outside of the central sense region in a mannersimilar to that of Krounbi et al above. Mowry's sensor strip is disposedbetween leading and trailing magnetic pole elements and the sensor stripis shielded from the trailing pole by a third shielding element.

In U.S. Pat. No. 5,014,147, Stuart S. P. Parkin et al. disclose anexchange-biased MR sensor strip employing an antiferromagnetic layercomposed of iron and manganese alloyed in specified proportions. Parkinet al. specify the Fe.sub.(1-x) Mn_(x) alloy, where x is within theinterval of [0.3, 0.4].

Clearly, numerous practitioners in the art employ NiFe/FeMnexchange-biased films in MR sensor assemblies for domain suppression. Indoing so, their exchange-bias field (H_(UA)) is typically applied alongthe length of the MR sensor element. The magnitude of H_(UA) must exceeda particular minimum to counteract demagnetization and coercivities inthe MR material.

For instance, a NiFe/FeMn exchange-biased film with, say, 400 Å (40 nm)of NiFe and 500 Å of FeMn provides an exchange-bias field of about 25Oersteds (2000 A/m). This 25 Oersted field is certainly adequate formost MR sensor geometries, especially for very long (i.e.: over 100micron) designs. Unfortunately, this exchange-bias field magnitudevaries sharply with changes in ambient temperature. For film thicknessesin the 400 Å/500 Å range, H_(UA) (T) varies from around 25 Oersteds atroom temperature to zero at a critical temperature T_(cr) of about 150°C. Because this variation is substantially linear, H_(UA) is reduced toonly 12 Oersteds at the maximum device operating temperature of 90° C.This 12 Oersted field value is generally only minimally sufficient toovercome the MR sensor coercivity of perhaps 10 Oersteds. There isaccordingly a clearly-felt need for exchange-biased film structures thatprovide a higher exchange-bias field value at H_(UA) at maximum deviceoperating temperature.

One approach for increasing the exchange-bias field H_(UA) at 90° C. isto introduce film changes to somehow increase H_(UA) at room temperature(20° C.) without incurring any significant reduction in criticaltemperature T_(cr).

The decline in H_(UA) at higher operating temperatures is even more of aproblem when using one of the corrosion-resistant FeMnX alloys (where Xincludes one of a group of elements including Cr, Rh and Ti) as theantiferromagnetic layer because H_(UA) is significantly lower at roomtemperature with these alloys. Thus, there is a clearly-felt need for atechnique that enhances H_(UA) in corrosion-resistant exchange-biased MRassemblies.

These unresolved problems and deficiencies are clearly felt in the artand are solved by this invention in the manner described below.

SUMMARY OF THE INVENTION

This invention increases the exchange-bias field H_(UA) in an MR sensorassembly at room temperature by adding a high-iron NiFe alloy transitionlayer to the interface region of the NiFe/FeMn system. This isaccomplished by increasing the iron/nickel ratio from the low-ironcontent preferred in the MR sensor strip to a high-iron content at thedirect atomic contact boundary. This avoids the disadvantageous effectsof increased MR element coercivity that would arise from merelyincreasing iron content throughout the MR strip.

It is an object of this invention to increase the exchange-bias fieldH_(UA) magnitude in the MR sensor strip at room temperature withoutreducing the critical temperature of the sensor assembly. It is anadvantage of the system of this invention that the exchange-bias fieldH_(UA) is increased by as much as 50% at room temperature withoutsignificantly affecting the critical temperature T_(cr).

It is another object of this invention to increase H_(UA) magnitude inMR sensors using corrosion-resistant antiferromagnetic layers. It is anadvantage of this invention that adding the same optimized ferromagneticlayer improves exchange-biasing sufficiently to increase H_(UA) at roomtemperature even when using the less-effective corrosion-resistantantiferromagnetic material. It is yet another advantage of thisinvention that the additional optimized interface layer providessubstantially the same exchange-bias field strength withcorrosion-resistant antiferromagnetic material as is available from themore effective antiferromagnetic material without the additionalferromagnetic layer.

The foregoing, together with other objects, features and advantages ofthis invention, will become more apparent when referring to thefollowing specification, claims and the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of this invention, reference is nowmade to the following detailed description of the embodiments asillustrated in the accompanying drawing, wherein:

FIG. 1 is an end view of a normal exchange-biased embodiment of amagnetoresistive (MR) sensor from the prior art;

FIG. 2 is an end view of an inverted exchange-biased embodiment of a MRsensor from the prior art;

FIG. 3 is an end view of a normal embodiment of the MR sensor assemblyaccording to this invention;

FIG. 4 is an end view of an alternate inverted embodiment of the MRsensor assembly according to this invention;

FIG. 5 is a graph of normalized exchange-bias field H_(UAN) as afunction of the iron content of the ferromagnetic MR layer;

FIG. 6 is a graph of normalized exchange-bias field H_(UAN) as afunction of operating temperature for several combinations of materials;

FIG. 7 is a graph of critical temperature T_(cr) as a function of theiron content of the ferromagnetic MR layer;

FIG. 8 is a graph of normalized exchange-bias field H_(UAN) as afunction of the iron content of the ferromagnetic MR layer for thestandard and inverted embodiments of FIGS. 3 and 4; and

FIGS. 9A-9B show a functional schematic illustration of a Direct AccessStorage Device (DASD) employing the MR sensor assembly of thisinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a typical thin-film magnetoresistive (MR) sensor from theprior art deposited on a substrate 10. A transverse biasing layer 12 isfirst deposited. Next, a non-magnetic spacer layer 14 is depositedfollowed by the MR layer 16, which is formed of a ferromagnetic materialsuch as Ni₈₀ Fe₂₀. Finally, an antiferromagnetic layer 18 is depositedin intimate contact with MR layer 16 to form an exchange-biasedinterface between layers 16 and 18. MR layer 16 is attached to the twoelectrical conductors 20 and provides therebetween the sensingresistance, which modulates a sensing current through MR layer 16. Theoutput current from MR layer 16 is a signal that enables a separatesensing circuit (not shown) to determine resistance changes in MR layer16. These resistance changes are a (usually linear) function of themagnetic fields intercepted by MR layer 16 from the recorded data on amagnetic storage medium (not shown) in the manner well-known in the art.

Transverse bias layer 12 provides a magnetic field oriented generallyperpendicular to the storage medium (not shown) so as to bias themagnetic field in MR layer 16 in a direction non-parallel to the storagemedium. This transverse bias field maintains the MR layer 16 in a linearresponse mode so that the output current is essentially a linearfunction of the change in resistance arising from incident magneticfields. As is known in the art, the transverse bias field can beprovided by shunt biasing, soft film biasing or permanent magnetbiasing.

To ensure that MR layer 16 has unidirectional anisotropy,antiferromagnetic layer 18 is disposed in direct atomic contact with MRlayer 16. Antiferromagnetic layer 18, which in the prior art may be thegamma phase of a manganese alloy, creates an interface exchange couplingwith the ferromagnetic material in MR layer 16. This results in alongitudinal exchange bias field in MR layer 16 sufficient to create asingle- domain state in MR layer 16. The single-domain state of MR layer16 is necessary to suppress Barkhausen noise, which is associated withMR materials having multiple magnetic domain states. A morecomprehensive description of the unidirectional surface anisotropyassociated with exchange coupling is provided in the above-citedHempstead, et al. patent.

FIG. 2 shows an alternate inverted MR sensor embodiment from the priorart in which a face-centered cubic (FCC) structure is provided by anauxiliary layer 22 such as copper or palladium. Auxiliary layer 22permits deposition of antiferromagnetic layer 18 before depositing MRlayer 16 because the proper FCC structure normally provided by MR layer16 is instead provided by auxiliary layer 22, which in a specificembodiment is 0.1 microns of metallic copper. In FIG. 2, spacer layer 14and transverse bias layer 12 are deposited after deposition of MR layer16. Hempstead et al. teach that, in the absence of a FCC structure uponwhich to deposit antiferromagnetic layer 18, no exchange coupling occursbetween layers 18 and 16. Independent of processing conditions, the biasfield H_(UA) in MR layer 16 peaks at an antiferromagnetic layer 18thickness of about 100 Å and falls off rapidly with increasing thicknessrelated to the structural transformation of antiferromagnetic layer 18.For lower and higher thicknesses of antiferromagnetic layer 18, theexchange coupling field H_(UA) is substantially similar to the fieldstrength obtained using the embodiment of FIG. 1.

This invention arises from the unexpectedly advantageous observationthat the exchange bias field magnitude H_(UA) depends on both layers 16and 18 while the temperature-dependence of H_(UA) (T) depends primarilyon only antiferromagnetic layer 18, which has a weaker magnetic orderthan MR layer 16. MR layer 16 consists of two atomic species, nickel andiron, and generally these two species exhibit different strengths ofexchange-coupling to antiferromagnetic layer 18. This difference isexhibited in FIG. 5, which shows the results of a series of experimentalstudies by the inventors of the thin-film structure of this inventionshown in FIG. 3.

FIG. 3 represents the prior art structure of FIG. 1 with the additionaltransition layer 24 of this invention deposited at the interface betweenlayers 16 and 18. Transition layer 24 may be fabricated over a range ofnickel and iron composition ratios. FIG. 5 shows the normalizedexchange-bias field H_(UAN) as a function of the iron content of theNiFe alloy in transition layer 24. The curve 26 represents experimentalmeasurements of the structure in FIG. 3 wherein layer 18 is 200 Å ofFeMn, layer 16 is 400 Å of NiFe and transition layer 24 is 50 Å ofNi.sub.(1-x) Fe.sub.(x), where x is the percentage of iron in thenickel-iron alloy. Curve 28 in FIG. 5 represents the same structuredescribed for curve 26 except the antiferromagnetic layer 18 alloy iscomposed of FeMnCr(3%).

The curves in FIG. 5 confirm that the iron in layer 24 is significantlymore effective than the nickel in coupling to the antiferromagneticlayer 18. For example, the 0% iron (pure nickel) alloy shows only 60% ofthe H_(UAN) found for Ni₈₀ Fe₂₀ alloy and the Ni₅₅ Fe₄₅ alloy showsalmost 150% of the exchange bias field H_(UAN) of the Ni₈₀ Fe₂₀ alloy.However, the Ni₄₀ Fe₆₀ alloy is less useful than the Ni₅₅ Fe₄₅ alloy,for unknown reasons. The inventors suspect that the Ni₄₀ Fe₆₀ material(pressed powder) was of poor quality compared with the other vacuum castalloys used in the experimental measurements, but the unexpected resultmay also arise from a basic weakening of the ferromagnetic order as thetransition layer 24 material approaches the non-magnetic Ni₂₅ Fe₇₅composition.

FIG. 6 shows the measured temperature dependence of the exchange-biasfield H_(UAN) for the several materials discussed above. The curves 30,32 and 34 in FIG. 6 represent the materials and characteristicssummarized below in Table 1.

                  TABLE 1                                                         ______________________________________                                        FIG. 6 Curve                                                                            Percent Fe  H.sub.UAN @ 20° C.                                                                 T.sub.cr                                    ______________________________________                                        30         0%         12.4 Oe     168° C.                              32        20%         21.2 Oe     183° C.                              34        45%         30.4 Oe     182° C.                              ______________________________________                                    

Except for the pure nickel material (curve 30), FIG. 6 shows that thecritical (or "blocking") temperature T_(cr) remains substantiallyunchanged over a range of ferromagnetic transition layer 24compositions.

FIG. 7 shows the relationship between critical temperature T_(cr) andthe percentage of iron in transition layer 24 of FIG. 3. Curves 26 and28 in FIG. 7 represent the analogous layer 16 and 18 materials describedabove in connection with curves 26 and 28 of FIG. 5. Note that criticaltemperature T_(cr) is essentially independent of transition layercomposition, other than for the pure nickel material.

FIGS. 5-7 show that a high iron content at the interface between theferromagnetic and antiferromagnetic layers in an exchange-biased MRsensor assembly offers the unexpected advantages of improvedexchange-bias H_(UAN) magnitude at room temperature without significantchange in the critical temperature for which H_(UA) =0. Accordingly, theMR sensor assembly of this invention includes a specially optimizedferromagnetic transition layer 24 at the interface region between MRlayer 16 and antiferromagnetic layer 18. With transition layer 24, theinterface coupling between layers 18 and 16 is significantly enhancedwithout materially affecting the temperature characteristics of theexchange bias field H_(UAN). This simple and direct approachunexpectedly increases the exchange-bias magnitude in all MR sensorapplications.

For instance, in FIG. 3, transition layer 24 is introduced as part of MRlayer 16 by increasing the percentage of iron in the nickel-iron alloyat the interface with antiferromagnetic/ layer 18. Similarly, in FIG. 4,transition layer 24 is introduced between layer 16 and 18 in the"inverted" configuration discussed above in connection with FIG. 2. Ineither FIG. 3 or FIG. 4, transition layer 24 can be optimized such thatthe iron-to-nickel ratio maximizes the exchange bias field H_(UAN) MRlayer 16.

FIG. 8 shows the results of additional experimental measurements usingthe transition layer technique of this invention discussed above inconnection with FIGS. 3 and 4. In FIG. 8, curve 36 shows therelationship between exchange-bias field H_(UAN) magnitude and thepercentage of iron in the ferromagnetic transition layer 24 of FIG. 3.Similarly, curve 38 shows the same relationship for the invertedconfiguration of FIG. 4. The measurements shown in FIG. 8 employed thefollowing materials:

Layer 14: Glass;

Layer 16: 400 Å of 80-20 NiFe;

Layer 24: 40 Å of Ni.sub.(1-x) Fe.sub.(x) where x=[0,0.6];

Layer 18: 200 Å of Mn₅₀ Fe₅₀ ;

Layer 22: 1000 Å of metallic copper; and

Closing layer (not shown): 200 Å of metallic tantalum.

Curve 38 in FIG. 8 shows that the inverted configuration of FIG. 4yields significant exchange-bias enhancement of up to 200% withinsertion of the 40 Å transition layer 24 of this invention.

The high iron content NiFe alloy is only one example of a ferromagneticlayer that better couples with the antiferromagnetic layer than does theusual Ni₈₀ Fe₂₀ alloy. Other ferromagnetic materials may also be used toprovide as good or perhaps better exchange coupling than the enhancednickel-iron alloys discussed above. For instance, ferromagnetic filmscontaining alloys of nickel with iron and perhaps manganese, cobalt,chrome, palladium and other magnetic species should be useful fortransition layer 24 in FIGS. 3-4.

Also, by introducing transition layer 24, the corrosion-resistant MRsensor assembly employing FeMnCr(3%) as antiferromagnetic layer 18 canalso be improved as shown in curve 28 of FIG. 5. If Ni₄₀ Fe₆₀ is used inMR layer 16 together with FeMnCr(3%) as antiferromagnetic layer 18, theroom temperature exchange bias field H_(UAN) of 20 Oersteds nearlyequals the 21 Oersteds obtained for the typical Ni₈₀ Fe₂₀ MR layer 16with the more active FeMn antiferromagnetic layer 18 (curve 26 in FIG.5).

The method and apparatus of this invention may be combined with otheruseful MR sensor improvement techniques known in the art. For instance,increased ferromagnetic biasing performance may be obtained not onlyfrom direct thin-film deposition procedures but also by the appropriatethermal treatments of the original exchange-coupled films such asNiFe/FeMn. Various such procedures are discussed above in connectionwith the existing art. Also, where 80-20 Permalloy material is notabsolutely required for the usual exchange-biased MR sensor system, theentire MR ferromagnetic layer 16 may be replaced by a ferromagneticmaterial specially optimized for exchange-coupling to theantiferromagnetic layer 18. Normally, the 80-20 NiFe alloy is preferredbecause of its lower (10 Oersted) coercivity, which must be overcome bythe exchange-bias field. However, through the use of otherantiferromagnetic materials and optimized exchange-coupling, thehigher-coercivity MR layer 16 material may be also advantageously usedin conjunction with the method of this invention.

The magnetoresistive sensor of this invention is suitable for use withmagnetic data storage devices such as the Direct Access Storage Device(DASD) illustrated in FIG. 9B. FIG. 9A shows a data storage disk 40disposed adjacent to an actuator arm 42, which includes a MR sensorassembly 44. FIG. 9B shows a DASD 46 in schematic form, including datastorage disk 40 and two actuator arms 42A and 42B, each containing MRsensor assembly 44A and 44B, respectively. DASD 46 includes a controlunit 48 that coordinates the mechanical motions of a drive motor 50 andan actuator motor 52. Drive motor 50 rotates disk 40 and actuator motor52 translates actuator arms 42A-42B to position heads 44A-44B radiallywith respect to disk 40. MR-sensor assemblies 44A and 44B operate toread magnetically-encoded data stored on the surfaces of disk 40 andthese data are transferred through a read/write channel 54 to controlunit 48 in a manner well-known in the art.

Clearly, other embodiments and modifications of this invention willoccur readily with those of ordinary skill in the art in view of theseteachings. Therefore, this invention is to be limited only by thefollowing claims, which include all such embodiments and modificationswhen viewed in conjunction with the above specification and accompanyingdrawing.

We claim:
 1. A magnetoresistive (MR) read transducer assemblycomprising:a thin MR layer of ferromagnetic material comprising an alloyof iron; a thin biasing layer of antiferromagnetic material; and aferromagnetic transition region disposed between said MR layer and saidbiasing layer, said ferromagnetic transition region comprising an alloyof iron and having first and second opposing sides, said first sideabutting the biasing layer at an interface therebetween, said secondside abutting the MR layer, wherein iron percentage in saidferromagnetic transition region is elevated with respect to ironpercentage in the MR layer.
 2. The MR read transducer assembly of claim1 wherein:iron percentage in the transition region varies from an uppervalue at said first side to a lower value at said second side.
 3. The MRread transducer assembly of claim 2 wherein:said upper value is between40% and 60%.
 4. The MR read transducer assembly of claim 3 wherein saidantiferromagnetic material is an alloy of iron and manganese.
 5. The MRread transducer assembly of claim 4 wherein said antiferromagneticmaterial is a FeMnX alloy, where X is an element selected from a groupcomprising chrome (Cr), rhodium (Rh) and titanium (Ti).
 6. The MR readtransducer assembly of claim 2, wherein iron percentage in thetransition region varies continuously between said upper value and saidlower value as a function of distance from the biasing layer.
 7. The MRread transducer assembly of claim 1 wherein:said lower value is between10% and 30%.
 8. The MR read transducer assembly of claim 7 wherein saidantiferromagnetic material is an alloy of iron and manganese.
 9. The MRread transducer assembly of claim 8 wherein:said antiferromagneticmaterial is a FeMnX alloy, where X is an element selected from a groupcomprising chrome (Cr), rhodium (Rh) and titanium (Ti).
 10. The MR readtransducer assembly of claim 1 wherein said ferromagnetic transitionregion is less than one-fourth the thickness of the thin MR layer. 11.The MR read transducer assembly of claim 1 wherein the thin MR layer isabout 400 Å and the ferromagnetic transition region is about 50 Å.
 12. ADirect Access Storage Device (DASD) system for storing data, said DASDsystem comprising:data storage means for retaining a plurality ofmagnetic transitions representing said data; control means coupled tosaid data storage means for retrieving said data from and storing saiddata to said data storage means; and a data sensor assembly coupled tosaid control means for sensing said magnetic transitions, said assemblyincluding:a thin MR layer of ferromagnetic material comprising an alloyof iron, a thin biasing layer of antiferromagnetic material, and aferromagnetic transition region abutting said MR layer and said biasinglayer, said ferromagnetic transition region comprising an alloy of ironwherein iron percentage in the ferromagnetic transition is elevated withrespect to iron percentage in the MR layer.
 13. The DASD system of claim12 wherein iron percentage in said transition region varies from anupper value at a first side abutting the biasing layer to a lower valueat a second side opposite the first side.
 14. The DASD system of claim13 wherein the lower value is between 10% and 30%.
 15. The DASD systemof claim 14 wherein:said antiferromagnetic material is an alloy of ironand manganese.
 16. The DASD system of claim 15 wherein:saidantiferromagnetic material is a FeMnX alloy, where X is an elementselected from a group comprising chrome (Cr), rhodium (Rh) and titanium(Ti).
 17. The DASD system of claim 13 wherein:said upper value isbetween 40% and 60%.
 18. The DASD system of claim 17 wherein saidantiferromagnetic material is an alloy of iron and manganese.
 19. TheDASD system of claim 18 wherein said antiferromagnetic material is aFeMnX alloy, where X is an element selected from a group comprisingchrome (Cr), rhodium (Rh) and titanium (Ti).
 20. The DASD system ofclaim 13, wherein iron percentage in the transition region variescontinuously between said upper value and said lower value as a functionof distance from the biasing layer.